问ACF自定义块get_field()在前端显示null？

EN

add_action('acf/init', 'my_acf_init_block_types');
function my_acf_init_block_types()
{
    // Check function exists.
    if (function_exists('acf_register_block_type')) {
        // register a hero block.
        acf_register_block_type(array(
            'name'              => 'banner',
            'title'             => __('Banner'),
            'description'       => __('A custom hero block.'),
            'render_callback'   => 'my_acf_block_render_callback',
            'category'          => 'home-sections',
            'icon'              => 'cover-image',
            'keywords'          => array('basic', 'banner')
        ));
    }
}

function my_acf_block_render_callback($block)
{
    // convert name ("acf/hero") into path friendly slug ("hero")
    $slug = str_replace('acf/', '', $block['name']);
    // include a template part from within the "template-parts/block" folder
    if (file_exists(get_theme_file_path("/template-parts/blocks/content-{$slug}.php"))) {
        include(get_theme_file_path("/template-parts/blocks/content-{$slug}.php"));
    }
}


function wpdocs_mytheme_block_styles()
{
    wp_enqueue_style('admin-css', get_stylesheet_directory_uri() . '/build/css/styles.min.css', '', '0.0.3');
    wp_enqueue_script('admin-js', get_template_directory_uri() . '/build/js/scripts.min.js', array('jquery'), '0.0.3', true);
} 
add_action('enqueue_block_editor_assets', 'wpdocs_mytheme_block_styles')

<?php
  $title = get_field('main_title');
    $img = get_field('background_image');
    $button = get_field('button');
?>
<section id="banner-section" style=" background-image:linear-gradient( rgba(67, 74, 122, 0.5), rgba(67, 74, 122, 0.5)  ), url('<?= $img['url']; ?>');">
    <div class="container">
        <div class="row justify-content-center">
            <div class="col-12 col-lg-10 text-center">
                <h1 class="basic-slider-heading"> <?php echo $title ?> </h1>
                <?php
                if ($button) :
                    $button_url = $button['url'];
                    $button_title = $button['title'];
                    $button_target = $button['target'] ? $button['target'] : '_self';
                ?>
                    <button class="btn btn-shadow-malibu btn-bg-malibu-gradient bg-malibu-accent">
                        <a href="<?= $button_url ?>" target="<?= $button_target; ?>"><?= $button_title ?></a>
                    </button>
                <?php endif; ?>
            </div>
        </div>
    </div>
</section>